A photometer in an aeroplane or satellite, pointed down at the ocean or any other water body can receive light originating in four different ways: by scattering from below the surface, by reflection of skylight at the surface, by reflection of the direct solar beam at the surface and by scattering within the atmosphere (Fig. 7.1). Only the first of these four light fluxes, which we shall refer to as the emergent flux, contains information about the underwater light field and the composition of the aquatic medium. The essential problem, therefore, is to quantify the emergent flux in the presence of the other light fluxes.
The photometer used to measure the light flux above the water surface is a radiance meter with a narrow angle of acceptance. To avoid receiving the surface-reflected solar beam, the radiance meter can be directed at a part of the surface well outside the solar glitter pattern. A more difficult problem is that of accounting for that part of the measured radiance which originates within the atmosphere. Depending on wavelength, anything from 80 to 100%, typically —90%, of the radiance received by a satellite-borne radiance meter, directed at the ocean surface outside the
Sun glitter pattern, originates in atmospheric Rayleigh (air molecule) and aerosol (particle) scattering. Ways of correcting for this flux are considered in a later section. This problem is largely eliminated when measurements are taken from low-flying aircraft (see Fig. 7.8a).
Skylight also originates in atmospheric scattering. Correction for the contribution of reflected skylight to the measured upwelling flux can be lumped in with the total atmospheric scattering correction.485 An alternative procedure is to carry out the measurements at an angle of 53° to the vertical.234 Light reflected from a plane surface at this angle (Brewster's angle) is polarized and can be eliminated by placing a polarized filter oriented at right angles to the major axis of polarization over the aperture of the radiance meter. A consequence of viewing at this angle, however, is that the effective air mass through which the flux from within the water must travel is increased by a factor of 1.67:41 this would be a significant disadvantage in the case of a satellite or high-flying aircraft, but not in the case of a low-flying aircraft.
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